In a fuel vapor treatment device for an internal combustion engine, the fuel vapor in a fuel tank is led into a canister to be temporarily adsorbed by the same, and the fuel vapor adsorbed by the canister is then led into an intake system of an internal combustion engine through a purge valve together with fresh air introduced from a fresh air inlet opening, thereby suppressing the escape of fuel vapor to open air.
In fuel vapor treatment devices of the type mentioned hereinabove, if the piping of a purge line extending from the fuel tank to the purge valve through the canister is cracked or poorly sealed at the joint portion of the piping, leakage of the fuel vapor is induced, preventing the fuel vapor treatment device from exhibiting sufficient fuel vapor escape suppression performance.
There are known leak diagnostic devices. For example, Japanese Laid-open Patent Application (Tokkaihei) 6-173789, discloses a leak diagnostic device that diagnoses or checks whether there is a leak of fuel vapor from the purge line or not by the change in pressure. The device performs a leak down detection operation wherein a predetermined amount of engine negative pressure is introduced, the purge line is hermetically closed up, and a determination is made whether there is a leak based on the change in pressure of the purge line. After the hermetical closing-up of the purge line, the internal pressure changes due to continuous vaporization of fuel in the fuel tank. The negative pressure change caused by the leak down is measured, and the interior of the purge line is exposed to the atmospheric air. The purge line is hermetically closed again, and the increase of internal pressure in the piping caused by the atmospheric pressure is detected to perform a vapor monitoring treatment for measuring a production speed of the fuel vapor. Based on the result of the measurement, the pressure variation at the leak down is corrected to increase leak detection performance.